Here is the full list of all 40 Legend of Grimrock achievements. It takes around 35-40 hours to unlock all of the achievements on Windows.
Find all secrets
Complete the game with old school mode
Mix 50 potions
Find all treasures
Find 5 skulls
Collect and wear full set of Valor armor (6pieces)
Open all iron doors
Collect and wear lurker's wardrobe (4 pieces)
Collect and wear full set of chitin armor (4 pieces)
Find 50 secrets
Obtain level 50 in any skill
Perform 500 ranged weapon attacks
Perform 250 unarmed attacks
Throw a rock 100 times
Complete the game
Complete the game with hard difficulty setting
Collect and wear full set of plate mail (5 pieces)
Give Toorum a hand
Get into the Prison
Find Zhandul's Orb
Jump 25 times into a pit
Cast 500 spells
Solve the Checkered room puzzle
Find Dismantler
Find entrance to the Vault of Dismantler
Find the pie
Survive the invasion of white blobs
Kill 250 monsters
Travel 10000 tiles
Find 25 secrets
Kill your first ogre
Cast 250 Spells
Mix 10 potions
Kill a monster by dropping or teleporting on it
Survive the slimes
Backstab a monster
Perform 500 melee weapon attacks
Find 10 secrets
Cast 25 spells
Finish first level in under 4 minutes